This invention is in the field of optical interference filters, and is particularly concerned with a filter that is formable by incident coherent radiation and which may be selectively erased. In general, optical interference filters consist of layers of optical materials in which the layers differ in some optical property such as reflectance, refractive index, or density. A typical filter uses alternate layers of relatively high and low indices of refraction, each layer being one-quarter wavelength of the radiation with which one wishes to interfere. Another type of interference filter is the Lippmann filter, in which are contained partially reflecting layers spaced apart one-half wavelength of the radiation to be reflected. The Lippmann filter layers are formed photographically as follows: a medium with photosensitive particles therein (a photographic film) is exposed on one side to coherent radiation and a mirror is placed on the other side to reflect the radiation and set up standing waves in the medium. When the medium is developed, parallel and partially reflective layers form, spaced apart one-half wavelength of the incident radiation. The main problem with these filters is that they are permanent; once formed, they cannot be erased and reformed. Moreover, they are not formable in real time, but must be prepared in advance. The instant invention is able to overcome these disadvantages; it provides a filter which is formable in real time, which may be erased and reformed, and which is formed by the radiation which it is desired to block.